Acyl chloride

In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group -COCl. Their formula is usually written RCOCl, where R is a side chain. They are reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CH₃COCl. Acyl chlorides are the most important subset of acyl halides.

Where the acyl chloride moiety takes priority, acyl chlorides are named by taking the name of the parent carboxylic acid, and substituting -yl chloride for -ic acid. Thus:

When other functional groups take priority, acyl chlorides are considered prefixes — chlorocarbonyl-:

Lacking the ability to form hydrogen bonds, acid chlorides have lower boiling and melting points than similar carboxylic acids. For example, acetic acid boils at 118 °C, whereas acetyl chloride boils at 51 °C. Like most carbonyl compounds, infrared spectroscopy reveals a band near 1750 cm⁻¹.

The industrial route to acetyl chloride involves the reaction of acetic anhydride with hydrogen chloride. For benzoyl chloride, the partial hydrolysis of benzotrichloride is useful:

In the laboratory, acyl chlorides are generally prepared in the same manner as alkyl chlorides, by replacing the corresponding hydroxy substituents with chlorides. Thus, carboxylic acids are treated with thionyl chloride (SOCl₂),phosphorus trichloride (PCl₃), or phosphorus pentachloride (PCl₅):

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